Gain measurement of distributed antenna system (DAS) segments during active communications employing autocorrelation on a combined test signal and communications signal is disclosed. In one embodiment, a test signal is injected into one or more DAS segments in a DAS. The test signal power is measured at the input and output of the DAS segment(s) for which gain measurement is desired. The difference in power of the test signal between the input and the output of the DAS segment is the gain of the DAS segment. The frequency of the test signal is provided to be within the frequency band of the communications service signals supported by the DAS segment. To allow for gain measurement of a DAS segment during active communication periods when the DAS segment is actively transmitting communications service signals, autocorrelation is employed to separate the test signal from combined test signal and communications service signals.